The immune system functions are one of the biological function systems most essential to understanding the phenomenon of life. Essentially divided into self and non-self, the immune system is a biological defense mechanism with more than a trillion cells that control the maintenance of the living body by complex and dynamic processes, and without a tissue structure. However, because of this complexity and subtlety, failure of these functions generates a variety of diseases, including therapeutically difficult diseases. It is important to understand the inflammatory reactions in infectious diseases, auto-immune diseases accompanying tissue destruction, and rejection during transplantation.
Antigen invasion into cells in the first stages of inflammation is nonspecific, or occurs via antibody and complement receptors. Antigens taken up into cells are processed into peptide fragments, and presented to T cells as a complex with major histocompatibility complex (MHC) class II molecules. In these antigen presenting cells (APCs) are dendritic cells, macrophages, B cells, Langerhans cells, interdigitating cells, monocytes in peripheral blood, and so on; that is, the so-called professional APCs. Depending on differences in the expression levels of the coreceptor molecules CD80 and CD86, and differences in the type of cytokines that are produced, a bias will arise in subsequent helper T cell (Th cell) divisions. Th2 types, which are mainly concerned with humoral immunity under the influence of IL-4, are dominant over Th1 types, which play a large role in cellular immunity under the influence of interleukin-12 (IL-12). In this way, primary effector cells are activated, or antibody production is induced, and sensitization is established. At the same time, memory T cells and memory B cells are produced. Primary effector cells, starting from Th1 cells, include cytotoxic T lymphocytes (CTL), mast cells, monocytes, macrophages, basophils, neutrophils, NK cells, platelets and the like. The action of cytokines, chemokinds, chemical mediators and such, which are produced by the activation of primary effector cells, activates vascular endothelial cells in the inflamed area. The activation of secondary effector cells, such as monocytes, macrophages, neutrophils, eosinophils and so on, is induced, and inflammation arises. Finally, the inflammation reaction is suppressed by phagocytosis of causative substances or effector cell apoptosis by Fas/Fas-L at the inflamed region, and transduction of immunoreceptor tyrosine-based inhibitory motif (ITIM) inhibition signals by CTLA-4 and FcγRIIB and so on.
The onset mechanism of autoimmune diseases is still unknown, however, research is progressing into where the cause of the above-mentioned process may lie. Despite the fact that in inflammation induction in post-organ transfer rejection and graft-versus-host-disease (GVHD) is very clearly caused by the invasion of foreign antigens, these diseases are still not completely controlled.
Other than Th cells and CTL cells, as mentioned above, auto-antibody production and a variety of effector functions of complements, cytokines and the like contribute to the inflammation reactions accompanying tissue destruction that occurs with immune abnormalities such as autoimmune disease and rejection, GVHD and so on. To control these effector functions for disease therapies, a number of immunesuppression therapies have been attempted. Many of these targeted T cells, and thus, the focus in controlling immune abnormalities is becoming control of effector T cells. Therefore, the study of T cells as inflammation effector cells can be said to be extremely important for the treatment of immune abnormalities.
Based on the above, the present inventors focused on studies of CD26. CD26 positive T cells are Th1 type cells, a subset that very easily migrates to the inflamed region. They contribute to autoimmune diseases such as rheumatoid arthritis, and immune abnormalities such as rejections and GVHD, and are known to accumulate in diseased regions. Thus, by furthering the understanding of CD26 positive T cells, more pathology-specific therapeutic methods can be established.
CD26 is a widely distributed 110-kDa cell surface glycoprotein consisting of 766 amino acids with known dipeptidyl peptidase IV (DPPIV, EC3.4.14.5) activity in its extracellular domain (Morimoto, et al., 1998; von Bonin et al., 1998). This enzyme is capable of cleaving amino-terminal dipeptides with either L-proline or L-alanine at the penultimate position. The expression of CD26 is enhanced after activation of T cells in a resting state. In addition, the CD4+CD26high T cells respond maximally to recall antigens such as tetanus toxoid (Morimoto, et al., 1989). Accumulating evidence suggests that DPPIV enzyme activity plays a role in the immune response (Oravecz et al., 1997; Iwata, et al., 1999). Crosslinking of CD26 and CD3 with solid-phase immobilized monoclonal antibodies (mAbs) can induce T cell costimulation and IL-2 production by either human CD4+ T cells or Jurkat T cell lines transfected with CD26 cDNA (Tanaka, et al., 1992; Fleischer, et al., 1994). In addition, anti-CD26 antibody treatment of T cells leads to a decrease in the surface expression of CD26 via its internalization, and such modulation results in an enhanced proliferative response to anti-CD3 or anti-CD2 stimulation as well as enhanced tyrosine phosphorylation of signaling molecules such as CD3ζ and p56-Lck (Hegen, et al., 1997). Moreover, DPPIV enzyme activity is required for the CD26-mediated T cell costimulation (Tanaka, et al., 1993). A recent report showed that internalization of CD26 after crosslinking is mediated in part by the mannose-6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR), and that the interaction of CD26 and M6P/IGFIIR plays a role in CD26-induced T cell costimulation (Ikushima, et al., 2000).
Maximal T cell activation requires both an antigen (Ag)-specific stimulus provided by an MHC peptide complex and a costimulatory signal (Lenschow, et al., 1996). Engagement of CD28 on the surface of T cells by B7-1 (CD80) or B7-2 (CD86) expressed on antigen presenting cells (APC) provides a potent costimulatory signal (Yokochi, et al., 1982; Azuma, et al., 1993; Freeman, et al., 1993; Lenschow, et al., 1996; McAdam, et al., 1998). CD28-B7 interactions lead to T cell proliferation, differentiation, and cytokine secretion (McAdam, et al., 1998; Chambers, 2001). In contrast, engagement of CTLA-4 on activated T cells by B7-1 or B7-2 results in an inhibition of T cell responses (Croft, et al., 1992; Walunas, et al., 1994; Krummel, et al., 1995). However, only CD28 is constitutively expressed, and hence it has an important role in the generation of T cell immune response (Fraser, et al., 1992; Caux, et al., 1994; Hathcock, et al., 1994; yi-qun, et al., 1996; Hakamada-Taguchi, et al., 1998; Manickasingham, et al., 1998).
Recombinant soluble CD26 (rsCD26) reportedly enhanced proliferative responses of peripheral blood lymphocytes (PBLs) to stimulation with the soluble antigen tetanus toxoid (TT) (Tanaka, et al., 1994). Amore recent report demonstrated that the target cells of rsCD26 were the CD14 positive monocytes in the peripheral blood, and that rsCD26 could upregulate CD86 expression, but not CD80 or HLA-DR antigen levels on monocytes (Ohnuma, et al., 2001). M6P/IGF-IIR is thought to be one of the platform molecules for CD26 interaction with APC. However, while both DPPIV-positive and DPPIV-negative rsCD26 were taken up by monocytes via M6P/IGF-IIR, only DPPIV-positive rsCD26 displayed an effect of CD86 upregulation on monocytes, thus suggesting that additional factors may interact with CD26 to directly induce CD86 upregulation on monocytes. Moreover, the molecular mechanism for the maximal response of CD4+CD26high T cells to the memory antigens has not yet been clarified.
Caveolin-1 is the primary coat protein of caveolae, and is involved as a regulator of signal transduction through binding of its scaffolding domain to key signaling molecules in various cells (Smart, et al., 1999; Peiro, et al., 2000; Carver, et al., 2003). Although CD26 was present in caveolae of fibroblast-like synoviocytes (Riemann, et al., 2001), its direct binding or signaling event was not demonstrated in immune cells.